The present invention relates to an electrophotographic image forming apparatus, and relates in particular to a configuration of a transfer device which transfers an image formed on an image carrier onto paper.
A transfer technique based on a corona charger facing a photosensitive drum is best known as a prior art of a transfer unit in an electrophotographic image forming apparatus. This method, however, has a problem of toxic ozone production. Thus, a contact type transfer technique is known as an ozoneless transfer technique.
Jpn. Pat. Appln. KOKAI Publication No. 6-110343 discloses a technique in which a transfer is performed by use of a semiconductive transfer belt, and a transfer roller provided on a back surface of the transfer belt. A transfer bias is applied to the transfer roller to achieve the transfer.
The following methods are known in connection with a color image forming apparatus which forms an image with a plurality of toners including Y (yellow), M (magenta), C (cyan) and Bk (black).
(1) A method in which the toners of four colors are superposed on one photosensitive drum to form an image, and the image is batch-transferred.
(2) A transfer drum method in which a transfer material is held on a transfer drum, and images of four colors are formed by four revolutions of the transfer drum.
(3) An intermediate transferrer method in which images of four colors are formed on an intermediate transferrer, and the image is batch-transferred to a transfer material.
(4) A four-drum method in which four photosensitive drums are disposed in parallel, and an image in four colors is formed while the transfer material passes one time.
Among the four methods described above, the intermediate transfer method (3) includes: (a) four revolution method in which one image carrier forms an image of each color, and repeatedly transfers the images onto the intermediate transferrer four times, and finally batch-transfers them on paper or the like; and (b) a four tandem method in which toner images of four colors are formed on an intermediate transfer belt at one time, and are finally batch-transferred on paper or the like.
In some of the intermediate transfer methods, a secondary transfer roller (transfer to the intermediate transferrer is a primary transfer) is used for a part where the final transfer is performed in the transfer material.
In the intermediate transfer method described above, a “fog” toner on the photosensitive drum sometimes sticks to the secondary transfer roller via the intermediate transferrer, in the image forming apparatus using the secondary transfer roller. Also, when the paper is not conveyed to a secondary transfer unit due to a problem in paper conveyance, the toner image on the intermediate transferrer sticks onto the secondary transfer roller. In such a case, there is a problem that the secondary transfer roller is smeared with the toner. The smeared secondary transfer roller leads to another problem that a rear side of the paper output at the next printing is smeared with the toner.
A technique is known in which a cleaner is attached to the secondary transfer roller, in order to solve the problem. Jpn. Pat. Appln. KOKAI Publication No. 2001-312154 discloses a configuration in which a secondary transfer member and a brush cleaning member are integrally brought in or out of contact.
Jpn. Pat. Appln. KOKAI Publication No. 2002-91191 discloses a technique for a configuration that satisfies 5×103≦(R1/R2)×(V2/V1)×D≦6×105 where R1 (mm) is an exterior angle of the secondary transfer roller, V1 (mm/sec) is a peripheral velocity thereof, R2 (mm) is an external diameter of a brush roller, V2 (mm/sec) is a peripheral velocity thereof, and D (number/inch2) is a hair transplant density.
However, in the configuration where the secondary transfer roller and the cleaning brush contact each other and rotate at an equal velocity and direction at the contact point, there is a problem of additional smearing due to a smear on the brush sticking again to the secondary transfer member.